The measurement of soil moisture at various depths in the soil is an increasingly important activity in modern agriculture. Crop yields are improved, water and fertilizer are conserved, and diseases are prevented when irrigation water is applied in a manner that avoids stress to a crop. One cause of crop stress is allowing the soil to become too dry. Other stress factors are root asphyxiation and excessive leeching of nutrients due to over-watering. Soil moisture measurements taken at appropriate soil depths, especially when taken within the root zone of the irrigated crop, provide necessary feedback to facilitate precise scheduling of irrigation.
A variety of sensors have been developed to detect moisture in soil. These include conductivity sensors and sensors of bulk dielectric constant. Methods used for measuring the dielectric constant include time domain reflectometry or transmissometry, frequency domain reflectometry (FDR), capacitance probe (CP), and ground-penetrating radar (GPR). These methods exploit the high dielectric constant of water relative to that of the medium being measured in order to extrapolate the moisture content of the medium.
Soil permittivity measurements have become the standard means of deriving soil moisture content. Time Domain Transmissometer (TDT) devices have been developed for use in soil moisture studies. Recent advances using low cost digital signal processing in conjunction with these devices has yielded high accuracy and stability even in the presence of temperature variations and moderate concentrations of salts and other ionic material in the soil. A TDT type of sensor such as that disclosed in U.S. Pat. No. 6,831,468 to Anderson, et al. ('468) is well suited for permanent installation where continuous moisture monitoring is needed for closed-loop irrigation control.
Typical sensing devices must be calibrated for the soil, and the readings must be interpreted by someone trained in the use of the specific sensing device. However, a sensor such as the TDT of the '468 reports absolute soil moisture at any desired soil depth so that it does not need to be calibrated for the soil. Its readings are stable with changing soil temperatures, electrical conductivity and compaction. The resulting data may be easily and reliably used by the crop grower without need of a consultant to interpret the readings. The data is of sufficient accuracy and stability to enable automatic, closed-loop irrigation scheduling.